This invention relates to an economical one-step decarboxylation process to substitute cyano radicals for silver carboxylate radicals using silver salts of aromatic and heterocyclic carboxylic acids.
Usually a decarboxylation reaction results in the generation of carbon dioxide and the concurrent replacement with hydrogen on the molecule. For example, the decarboxylation of benzoic acid yields benzene and carbon dioxide. Pyridine carboxylic acid goes to pyridine and carbon dioxide. The presence of other carboxylic acid salts such as a sodium carboxylate of an aromatic acid will merely aid in the decarboxylation reaction and char to yield an ill-defined residue at best. However, the decarboxylation products using silver salts of carboxylic acids are silver, carbon dioxide and the coupled organic radicals of the silver carboxylates in the form of dimers, trimers and multiples thereof. Surprisingly also, if the decarboxylation reaction takes place in the presence of silver cyanide, the decarboxylation process forms organic nitriles of the reactant silver salt in a one-step process. The individual resulting components are readily separable by distillation due to the great differences in boiling points.
Many procedures exist for making nitriles from carboxylic acids or esters which mainly include different dehydration catalysts, temperatures and pressures. The usual process involves the conversion of the acid or ester to the amide, using an excess of ammonia which is then dehydrated to the nitrile. For example, U.S. Pat. No. 2,794,043 teaches the preparation of nitriles by reacting aliphatic acids containing 8 to 20 carbon atoms with 0.2-0.8 mole excess ammonia in the presence of a dehydration catalyst such as Al.sub.2 O.sub.3, silica gel, or acid clays in the liquid phase at 200.degree.-300.degree.C, while removing the water from the reaction mixture by vaporization. The mixture is then subjected to vapor phase conditions at 300.degree.-400.degree.C in the presence of a dehydration catalyst and ammonia. This type of process is adaptable to continuous production.
A well-known modification of the procedure making nitriles from carboxylic acids is ammoxidation. In this process, a hydrocarbon such as toluene or xylene is oxidized to the acid in the presence of ammonia and oxygen with a dehydration contact catalyst at high temperatures, 400.degree.-450.degree.C at 5-30 psig. The nitrile is produced directly without isolation of the acid. The ammoxidation of o-xylene has been reported (Y. Ogata and K. Sokanishi, Chem. Ind. (London) 1966, 2055-56). The catalyst used was 5% V.sub.2 O.sub.5 and Al.sub.2 O.sub.3 with K.sub.2 SO.sub.4 cocatalyst at 400.degree.C. But ammoxidation does not work if there are more than two methyl groups on the benzene ring, nor does it work with heterocyclic compounds. The ammoxidation conditions also argue for rapid oxidation with total destruction of easily oxidized materials.
It is, therefore, a general object of my invention to provide a new process for making aromatic and heterocyclic nitriles without the necessity of converting the acid to the amide before dehydration to the nitrile or being restricted to hydrocarbons with alkyl groups in the desired cyano radical position. Another object of my invention is to provide a practical and economic process for the manufacture of heterocyclic nitrile compounds. A further object of my invention is to provide a practical and economic process for the manufacture of polyphenyl nitriles directly from monobenzenoid compounds. The nature of still other objects of my invention will be apparent from a consideration of the descriptive portion to follow.
It is my discovery that the above and other objects of the invention are attained by the silver salt/silver cyanide process herein described. I have found that the silver salts of aromatic and heterocyclic acids react with silver cyanide at elevated temperatures to produce organic nitriles. This process also gives polyphenyl nitriles directly from monobenzenoid compounds. Many of these organic nitriles which can be produced by my invented process are commercially desirable and used in commercial quantities as solvents of high dielectric and thermal stability, and as chemical intermediates.